This is a great find. How interesting that a possible solution was right under our noses as a component of solar PV panel installation.

6mm X 0.2mm ribbon is equal to 1.2mm square wire. And that makes it equal to 16/17 AWG gauge wire. Thats fatter than needed for the parallel connections, but for the series? If there is one strip per cell (using common design of plastic cell holders), then...17-ga would be adequate for 3A continuous and 19A temporary peak? (per cell in the P-groups).

The tin-plating is for outdoor corrosion resistance, and it is very thin. The IACS conductivity of tin is 15/100, so...worse than the pure nickel at 22/100, but again...if its very thin there wouldn't be a lot of waste heat. The copper core would act as a heat-sink (thicker and wider would be better, of course). As a reminder, the copper core IACS conductivity is 100/100, and resistance is near zero if thick enough. So, 5-times as conductive as pure nickel (500% better)

[edit: pure nickel has a poor conductivity of 22/100, and copper is 100/100, but when copper busses are only plated with nickel, with the nickel acting only as a corrosion-resistance layer, the end result is rated as roughly 80/100. I suspect zinc-coated copper-strips would have a very similar result, and would be cheaper, plus easy to make at home]

What width of ribbon do most plastic cell-holders allow? 8mm? 10mm? (edit, apparently, 7mm wide nickel ribbon is the common standard)

edit: A quick google shows solar panel tinned bus strip can be had as tin-plated brass or copper. The tin-plating is VERY thin, but...I wonder if it is "just thick enough" to make spot-welding the ribbon onto the cell-end easy (tin has high resistance). Pic from Ulbricht ("bus wire for crystalline silicon")

Can somebody point me to a link or explain the proper way of soldering copper wire on to the nickel strip as shown in the above pic? I notice that the solder joints are between cells to minimize heat transfer to the cells. Even doing that, doesn't heat still, if somewhat reduced, still transfer to the cells?

Or perhaps did you pre-solder all the copper wiring onto the strip and then spot weld the strip onto the cells?

aethyr wrote:
Even doing that, doesn't heat still, if somewhat reduced, still transfer to the cells?

The heat that reaches the sensitive part of the cell is so little in this case, it isn't worth to think about it
From the welding, the "heat peak" is much higher as the steel get melted.
I also would say now that soldering directly on the cells (with professional tools for a very very short time) also doesn't harm, or at least not more than welding (especially on the can).

The main reason i solder between the cells is that direclty on the welds the solder doesn't hold or flow that good.

aethyr wrote:
Or perhaps did you pre-solder all the copper wiring onto the strip and then spot weld the strip onto the cells?

the sheets i beef up after welding them onto the cells, and i work from one side to the other step by step.
The area on the wire and sheet i couat with tin before soldering them together.

aethyr wrote:
Even doing that, doesn't heat still, if somewhat reduced, still transfer to the cells?

The heat that reaches the sensitive part of the cell is so little in this case, it isn't worth to think about it
From the welding, the "heat peak" is much higher as the steel get melted.
I also would say now that soldering directly on the cells (with professional tools for a very very short time) also doesn't harm, or at least not more than welding (especially on the can).

The main reason i solder between the cells is that direclty on the welds the solder doesn't hold or flow that good.

aethyr wrote:
Or perhaps did you pre-solder all the copper wiring onto the strip and then spot weld the strip onto the cells?

the sheets i beef up after welding them onto the cells, and i work from one side to the other step by step.
The area on the wire and sheet i couat with tin before soldering them together.

Thanks, I think that was your pic to begin with

I think I'll follow your approach of pre-tinning the strips. That way I don't have to heat the connection as much.

One more question: your copper bus wire - doesn't current still have to pass through the thin nickel strip to reach the copper wire, and so doesn't the strip remain a bottleneck for the current? The reason I ask is that I'm designing a high current pack and I want to ensure no bottlenecks.

aethyr wrote:
One more question: your copper bus wire - doesn't current still have to pass through the thin nickel strip to reach the copper wire, and so doesn't the strip remain a bottleneck for the current? The reason I ask is that I'm designing a high current pack and I want to ensure no bottlenecks.

Yes, but if the copper is right on top of the strip, the path distance through the nickel will be very short and not result in significant resistance or heating.

I've soldered directly to the ends of cells before and not had any issues. Soldering to the strips will heat the cells way less than direct soldering, but you need to pay special attention to the insulation under the strips. The fiber rings will prevent melt through.

Allex wrote:
Same what, length? If we consider thickness then the length is not that critical any more, right?

Same conductivity. (In simple: use same wire )
Example: If you take just 1S4P, for simplicity, as pictured and beef up the conductor on negative side, your current path resistance will differ for every cell in this parallel group.

Nothing wrong with spot-welding. Once the machine is dialed-in, any average garage builder can get consistent results. However, I had always felt in the past that soldering was a poor substitute. I now feel that...if the proper tools and techniques are used, soldering can be a very viable option (especially resistance soldering).

The bottom of an 18650 cell (negative anode) is the more sensitive end, and care should be taken to not damage that end with too much heat, regardless of the method used. Because of this, I now think that individual cell fuse-wire is not only useful, but it is (IMHO) the best connection method to the negative end. This includes spot-welded fuse-wire, or soldered fuse-wire.

The positive cathode can take quite a bit of heat without suffering any damage. Of course I do not recommend applying lots of heat, simply do the minimum necessary to make a solid connection. Copper ribbon is the best material for series connections, for carrying the main pack current. It can be found locally in every major city, and 0.25mm copper can easily be cut with stout scissors. If you live in a humid climate where you are concerned about corrosion, the copper can easily be nickel-plated in your garage.

Copper is very difficult to spot-weld with an expensive machine, and the common units cannot do it at all. However, if you experiment to verify the proper technique, soldering can reliably attach copper ribbon onto the positive cathode.

How much difference is this "worst current share" regarding current draw? I guess it depends but just wondering if it is more theory than real world problem? I was thinking of using N.E.S.E. module (see link below), and they look like the "worst current share" in the picture. So how bad is it? Each cell peak current 8A with Sanyo 3500mAh. Say 4P.

The modules itself can handle 50A with no problem, but will I have problem ragarding current path through cells?

This is what I was thinking about. Anyone have experience of this or is most of it in theory? Then ofcourse each pack and material would be different.
So the theory is that the extra cm of material resistance (compared to first cell) would be so high that first cell would take so much more current that it would decrease its life time?
Then of course if material is too thin there would be heat, but if material is enough it should only be material resistance?

I've read though all the posts in this thread and I am puzzled while I am in the process of building my second pack. Maybe someone can give me a couple of suggestions.

The first pack I've built was for my brother and it was a 48V 13S6P using cheap Chinese 18650s that had on-board protection. The connections were simple and easy using nickel strips in pairs all the way from cell 1 to cell 13. No BMS was needed as the batteries were protected and the final result was very cheap and perfect for a folding bike with a 500W motor and a 30' to 45' range. So, a success there with the use of nickel plates and simple wiring.

My latest project however, involves 195 unprotected 18650s (Again cheap Chinese ones - £160 for 200 of them) with a BMS to balance the series connection and a special parallel connection for current balancing. The final pack will be a 13S15P to power a 1500W rear hub motor. The intention is to put the pack inside the 40x50x60cm triangle of my bike, along with the BMS and the 4A charger. Basic calculations reveal that I will need a maximum of 31.25A to use the motor to its fullest. Since the cells will be connected in parallel first and then in series, my problem lies with the nickel strips connecting the individual cells in parallel.

Regarding my parallel connections, I am amazed that no one has mentioned this here before... please take a look at the following site http://www.smartgauge.co.uk/batt_con.html to see why most of the designs I've seen in this thread are actually under-performing! The site describes the 4P connection and the 15P is a bit more complicated but I have made that calculation already.

My biggest problem at the moment is how I can connect the cells with something that can allow 15A (30A is max but will split in half at the worst case scenario) without having temperature issues and have the least voltage drop at the same time. Ideally, I would like copper strips, but, as I understand from the discussion, I won't be able to spot weld copper strips using my simple £100 ebay spot welder.

I am at work at the moment, so, I will post my battery connection diagrams later today.

Any thoughts are welcome. I want to build a robust and long-lasting battery and use every little bit of those cheap Chinese cells!